Optical coherence tomography (OCT), also referred to as phase-variance optical coherence tomography, is one of the most powerful and more widespread biomedical imaging techniques. It has applications in several fields of medicine. The ophthalmologic field has greatly contributed to its development and optimization.
In this technique any information relating to the structure of the sample/organ being observed is derived from the radiation reflected back and/or backscattered from regions showing different optical properties within the sample/organ itself.
The OCT technique allows to create two-dimensional or three-dimensional models having a resolution of one to few μms. Besides allowing a morphological study, OCT may reveal other biological properties of the sample being analysed, such as for example flow rate (by means of the Doppler effect) and birefringence (by means of polarisation changes).
OCT has its bases in low-coherence interferometry. The optical set up of the OCT system is based on a Michelson interferometer and, the OCT system operating mode is determined depending on the type of radiation source and detection technique used. Currently, there are two main schemes used in OCT instruments.
In the so-called Time-Domain OCT (TD-OCT) the reflectivity profile of the sample is obtained by having the radiation coming from the sample optical arm interfere with that coming from the reference optical arm, whose path is modified within a certain time interval. The displacement of the reference arm is the measurement of the distance of the sample member that has caused the reflection.
The Fourier Domain OCT (FD-OCT), on the contrary, records in one step, without the need of a mechanical translation of the members in the reference arm, the spectrum fringes caused by the interference of the radiation coming from the sample arm with that coming from the reference arm, in a broad spectral band. The measurement of the distances of the various sample members is obtained by processing the interferogram signal.
The second technique is much faster than the first one in that it reduces the presence of moving parts and also has benefits in terms of signal-to-noise ratio which result in higher image quality.
In turn, the second FD-OCT technique may be applied according to two main embodiments:                Spectral Domain OCT (SD-OCT), wherein the spectrum is obtained by using a broadband radiation source and a spectrometer which measures its intensity with a linear sensor (line-scan camera);        Swept Source OCT (SS-OCT), wherein the spectrum is obtained by an individual radiation detector by making the wavelength emitted by the source vary at very high speeds.        
For the sake of clarifying the basic concepts underlying the invention, hereinafter reference will be made to a configuration of the SD-OCT type, but with obvious adjustments the man skilled in the art may readily extend the technique that will be illustrated to the other configurations referred to hereinabove and to known variations thereof.
With specific reference now to FIG. 1, which relates to a conventional SD-OCT configuration, the system comprises:                a broadband radiation source LBS;        a reference optical arm RA which contains a lens system L2 and a mirror Mref;        a sample arm SA which contains a scanning system, consisting of a lens system L1 and a mirror and actuator system M, which allows to illuminate a strip (in the axial direction) of the sample of which an image is to be generated and the backscattered radiation is to be collected;        a signal detection arm MA with a spectrometer Spec which allows to analyse the spectrum of the signal resulting from the interference of the radiation coming from the reference arm RA and from the sample arm SA, comprising a linear sensor detecting the spectrum of the interference signal corresponding to the illuminated strip of the sample;        a beam-splitter BS configured so that it allows the passage of the radiation from the source LBS to the sample arm SA and to the reference arm RA, and from these to the detection arm MA; and        a control and processing unit CUP which suitably controls the mechanical and electronic components, and derives from the spectrum, by means of one of the many algorithms known in the literature, a reflectivity profile of the sample strip an image of which is to be generated.        
The broadband light radiation source LBS is transmitted to the reference arm RA and to the sample arm SA opposite to which the sample to be imaged is placed. The radiation in the reference arm RA is reflected by the mirror MRef and is sent through the beam-splitter BS to the detection arm MA. Similarly, the radiation in the sample arm SA is backscattered from the illuminated sample portion and arrives through the beam-splitter BS to the detection arm MA. Therefore, the two light waves, coming from the reference arm RA and the sample arm SA, interfere on with the detection arm MA where the spectrometer Spec reconstructs on a linear sensor the spectrum of the interference signal (interferogram).
The above-mentioned spectrum is transformed by means of one of the algorithms known in the literature in the reflectivity profile of the illuminated sample portion. If, for multiple strips, it is possible to measure the reflectivity profile, a cutaway image of the sample may be obtained. From such a cutaway image measurements relating to the sample shape may be obtained. In the case of an eye, for example (see the illustration of FIG. 2), if the eye anterior segment is observed, the altimetrical profile and the curvature of the surfaces of the cornea, the crystalline lens and the iris may be obtained. If many images relating to different sample sections are captured, it may even be possible to generate a three-dimensional model of the sample.
If one decides to use a configuration according to the SS-OCT technique, the an skilled in the art may replace the broadband source with a source having an emitted wavelength that can be varied very quickly over time, and the spectrometer of the detection branch with a single detection channel radiation detector. In this case, the output signal spectrum is built by varying the wavelength emitted by the source and by sequentially storing the intensities measured by the detector for each wavelength.
In order to obtain an image of a section of the eye anterior segment, therefore a linear scan is generally performed and at the end the information obtained is processed into one single image. Then with reference to FIG. 3, if one assumes the use of just one mirror M for a two-dimensional scan, the scan is obtained by changing the inclination of the mirror in the sample arm and consequently the side position of the lighting beam coming from lens O. When the mirror is in position M′, the lighting beam R′ illuminates the central part of the scanning space and allows the detection of structures in that portion of the sample. When the mirror is in position M″, the lighting beam R″ illuminates the bottom part of the scanning space. When the mirror is in position M′″, the lighting beam R′″ illuminates the top part of the scanning space.
The illuminated tissue portion backscatters part of the radiation, with an angular scattering of the intensity that depends on its microstructure and the orientation of its discontinuity surfaces. In general such scattering, also referred to as lobe, will be uneven, with an intensity peak in the reflection direction, symmetrical to that of lighting as compared to the normal to said surfaces, and with decreasing intensity in the peripheral directions. The radiation that is actually collected for measurement is that which is backscattered exactly in the opposite direction to that of lighting. Such radiation, which returns to the instrument, will pass through the sample arm of the interferometer and will interfere in the detection arm with the radiation coming from the reference arm on the spectrometer branch.
By observing FIG. 4, it may be noted that, if the sample observed has a marked curvature, as in the case of an eye, the more the scan departs from the corneal vertex CV the lower the backscattered power will be in the incidence direction, because a large part of the backscattered energy will be deviated towards the reflection direction, away from that of incidence, and will therefore be lost and unavailable for the detection by the instrument. The above-mentioned figure clearly shows how increasing the incidence angle (angle between the incidence radius Inc1 or Inc2 and the normal to the anterior corneal surface Norm1 or Norm2) also increases the reflection angle and thus the deviation of reflected power (Ref1 or Ref2) towards a direction which is not useful for the instrument to detect a return signal.
A small part of the incident power will however be scattered in the other directions of the lobe, among which is also the one opposing the forward path. In the practice, a reduction in the power collected by the instrument is observed as the scan goes from the centre to the periphery.
In order to address this problem, Rahul Yadav et al. in the paper “Scanning system design for large scan depth anterior segment optical coherence tomography”—OPTICS LETTERS Vol. 35, No. 11/Jun. 1, 2010, suggest a particular configuration of the scanning system consisting of concave mirrors and lenses, which allows the scanning beams to get to the cornea almost as normal or, in other words, with a lower incidence angle. This certainly provides an increase in the quantity of radiation backscattered towards the instrument and results in an increased signal-to-noise ratio.
However, some significant shortcomings are also found. Firstly, the system is complex and expensive because it envisages the use of lenses and mirrors of a non-classical shape and further the various members must be spatially disposed at precise angles, which can make the alignment very demanding and difficult. Further, the system may be very bulky, especially if a quite wide working distance is selected, such that it reduces the discomfort for the patient and/or is needed for the coexistence of other integrated instruments with the one described.